Over these recent years, to cope with the depletion of fossil fuel and the need to stop global warming, the so-called energy harvesting (environmental power generation) is gathering attention and undergoing research and development. Energy harvesting uses natural energy, which is obtained through photovoltaic power generation, wind force power generation, and the like; piezoelectric/vibration powered generation; and human powered generation performed in living activities.
For example, power may be generated when the pushing of a button causes a mechanical switch to change inductance, increases the generated voltage to obtain a weak power, drive a high frequency wireless system, and radiate the weak electric wave to the vicinity to control the device.
An old and well known technique uses rotational kinetic energy of a bicycle tire to generate power, which is supplied to a lamp, with a power generator referred to as a dynamo generating capacity. The power generator generally has the same structure as a DC motor and uses a mechanism allowing for reversible electrical-mechanical energy conversion.
Further, a flash light including a power generator has been commercialized over these recent years. When a handle or a lever of the flashlight is operated and a rotor of the power generator is rotated, the flashlight generates power and activates an LED light.
Contactless power supply that supplies an electric device with power in a wireless manner has increased efficiency and become practical. Thus, it may be anticipated that the power obtained through energy harvesting be applied to contactless power supply.
When the power obtained through energy harvesting is applied to contactless power supply, for the contactless power supply to be practical, there is a need for the minimum output enabling at least an LED light, a buzzer, or the like to be driven (i.e., voltage of several V or greater input to a load and output obtaining power of 10 mW or greater).
Non-patent document 1 reports a study case in which the power supplied by a solar battery is stored in a rechargeable battery to perform contactless power supply using a stable DC voltage of 3 V is carried out. Non-patent document 1 shows, with a graph, the result in which the output voltage is about 4.5 V at a load of 180 ohms when the coil size of a primary coil (secondary coil also has the same dimension as the primary coil) is 38 mm×19 mm and the gap between coils is 2.6 mm.
The output power in this case is not described in non-patent document 1 but may be estimated through calculation as being 0.11 W (4.5×4.5/180).
The output power is the power enabling for the activation of an LED light or the like. However, the gap in this example is 2.6 mm and small. Accordingly, it may be understood that the system in this example does not enable the supply of power to a device separated by a large distance of a few centimeters to tens of centimeters. Non-patent document 1 uses a solar battery that supplies fluctuating power. However, the supplied power is temporarily stored in a rechargeable battery so that the rechargeable battery is able to supply stable voltage. The voltage supplied from the rechargeable battery is 3 V and low but sufficient for driving a circuit.